Effect of chronic inhibition of angiotensin converting enzyme on baroreceptor reflex in essential hypertension

The effects of preoperative therapy with angiotensin-converting enzyme inhibitors on clinical outcome after cardiovascular surgery

OBJECTIVE

To determine the effect of preoperative therapy with angiotensin-converting enzyme (ACE) inhibitors on clinical outcome after cardiovascular surgery.

STUDY

Inception cohort.

SETTING

A tertiary care 54-bed cardiothoracic ICU.

PATIENTS

All admissions to an ICU over a 42-month period after cardiovascular surgery.

INTERVENTION

Extraction of preoperative, operative, and ICU data from a database.

OUTCOME MEASURES

Incidence of acute organ dysfunction, length of mechanical ventilation, ICU stay, and death after cardiovascular surgery.

RESULTS

The study cohort consisted of four groups: normal or moderately impaired left ventricular function control (group A, n=6,400); normal or moderately impaired left ventricular function treated with ACE inhibitors (group B, n=1,375); severe left ventricular dysfunction control (group C, n=1,905); and severe left ventricular dysfunction treated with ACE inhibitors (group D, n=1,650). The incidence of three or more organ dysfunction was similar on comparison of group A vs group B (5% vs 6%) or group C vs group D (15% vs 13%). There were no differences in the total duration of mechanical ventilation or length of stay in the ICU in group A vs group B or group C vs group D. Death occurred in 2% of groups A and B, and at 6% in groups C and D. Preoperative severe left ventricular dysfunction in both groups C and D was associated with an increased incidence of three or more organ dysfunction, duration of mechanical ventilation, length of stay in ICU, and death after surgery. Multivariate analysis indicated that therapy with ACE inhibitors did not affect the clinical outcome after cardiovascular surgery.

CONCLUSION

Preoperative therapy with ACE inhibitors did not influence the clinical outcome after cardiac surgery. It is unlikely that therapy with ACE inhibitors can alter the clinical sequelae of cardiopulmonary bypass and cardiac surgical procedures performed in high-risk patients because of underlying severe left ventricular dysfunction.

Role of AT1 receptors in the resetting of the baroreflex control of heart rate by angiotensin II in the rabbit

Angiotensin II (Ang II) resets the baroreflex control of heart rate to a higher blood pressure. This action is apparently mediated via Ang II receptors in the area postrema, but it is not known if these are of the AT1 or AT2 subtype. In the present study the effects of losartan, a selective AT1 receptor antagonist, and PD 123319, a selective AT2 antagonist, on the cardiac baroreflex response to Ang II were investigated in conscious rabbits with chronically implanted arterial and venous catheters. Baroreflex curves were generated with intravenous infusions of phenylephrine and nitroprusside (2.6-25 micrograms/kg per min) and analyzed using a four-parameter logistic model to yield their upper and lower plateaus, arterial pressure at the midpoint of the heart rate range (BP50), and slope coefficient. From these four parameters, the gain and range of the baroreflex were calculated. Background intravenous infusion of Ang II at 10 ng/kg per min increased mean arterial pressure by 17 mmHg but did not change heart rate. Ang II shifted the baroreflex curve to the right as indicated by an increase in BP50 from 70.9 +/- 2.0 to 89.3 +/- 2.7 mmHg (P < 0.05), but did not change baroreflex gain significantly. Ang II did not alter the upper plateau of the baroreflex, but decreased the lower plateau from 119.4 +/- 10.3 to 73.6 +/- 11.5 beats per minute (bpm) (P < 0.05), extending the heart rate range by 52.5 bpm. Pretreatment with losartan completely abolished the pressor and cardiac baroreflex responses to Ang II. In contrast, PD 123319 had no effect on these responses. Administration of losartan alone to block endogenous Ang II shifted the baroreflex curve to the left as indicated by a decrease in BP50 from 71.2 +/- 2.7 to 64.7 +/- 2.5 mmHg (P < 0.05). These results demonstrate that the resetting of the baroreflex control of heart rate by Ang II is mediated by AT1 receptors, and that basal levels of endogenous Ang II exert a tonic action on the cardiac baroreflex to increase the setpoint around which the baroreflex regulates heart rate.